Implantable stimulation devices generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system. However, the present invention is applicable to other implantable medical device system, as will be discussed subsequently.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. As shown in FIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG) 10, which includes a biocompatible case 12 formed of a conductive material such as titanium for example. The case 12 usually holds the circuitry and power source or battery 25 necessary for the IPG to function, although IPGs can also be powered via external RF power and without a battery. The IPG 10 is coupled to electrodes 20 via one or more electrode leads (two such leads 16 and 18 are shown), such that the electrodes 20 form an electrode array 14. The electrodes 20 are carried on a flexible body 22, which also houses the individual signal wires 26 and 28 coupled to each electrode. The signal wires 26 and 28 are connected to the IPG 10 by way of an interface 35, which allows the leads 16 and 18 (or a lead extension, not shown) to be electro-mechanically or remotely (e.g. wirelessly) connected to the IPG 10. Interface 35 may comprise lead connectors 36 and 38 embedded in a non-conductive header 40, which can comprise an epoxy for example. The header 40 can further include a telemetry antenna or coil 42 for receipt and transmission of data to an external device such as a portable or hand-held external controller (not shown).
As illustrated, there are eight electrodes on lead 16, labeled E1-E8, and eight electrodes on lead 18, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. The electrode array 14 is typically implanted along the dura of the spinal cord, and the IPG 10 generates electrical pulses that are delivered through the electrodes 20 to the nerve fibers within the spinal column. The IPG 10 is typically implanted somewhat distant from the leads 16 and 18, such as in the upper portion of the patient's buttocks (see FIG. 3).
As shown in cross-section in FIG. 2, an IPG 10 typically includes a printed circuit board (PCB) 44 containing various electronic components 46, such as microprocessors, integrated circuits, and capacitors. Ultimately, the electronic circuitry performs a therapeutic function, such as neurostimulation. A feedthrough 49 routes the various electrode signals from the electronic circuitry to the lead connectors 36 and 38, which are in turn coupled to the leads 16 and 18 as mentioned previously.
Also shown in FIG. 2 is an external charger 50 that is used to power the IPG 10, commonly by recharging the battery 25 in the IPG 10. The external charger 50 itself needs power to operate, and therefore may include its own battery 52, which may also be rechargeable using a plug-in-the-wall holster (“cradle”) or power cord connection. Alternatively, the external charger 50 may lack a battery 52 and instead draw its power directly from being plugged into a wall outlet (not shown).
The external charger 50 can contain one or more PCBs 54, which contain the circuitry 56 needed to implement its functionality. The external charger 50 comprises a case or housing 58, typically formed of a hard plastic, which may be divided into top and bottom portions 58a and 58b. The case 58 can be hand-held, body-worn, and/or portable. Junction 59 illustrates the location where the top and bottom case portions 58a and 58b may be snapped together or connected by other means. Clamps 60 may be utilized to hold the PCB 54 and other internal structures in place.
The charger 50 typically includes an alternating current (AC) coil 62, which generates an AC magnetic field to supply power 64 to the IPG 10. The magnetic field induces an AC current in a charging coil 48 located in or on the IPG 10 via inductive coupling. This means of inductive power transfer can occur transcutaneously, i.e., through the patient's tissue 80. The power 64 received by the IPG's coil 48 can be rectified and used to recharge battery 25 in the IPG 10, which in turn powers the IPG 10. Alternatively, power 64 can directly power the IPG if it lacks a battery.
External charger 50 typically employs a relatively simple user interface 70, which simplicity is warranted because of the relative simplicity of the charging function, and because the external charger 50 may not be visible to the patient while in use, thus limiting the utility of more complex visual user interfaces. The user interface 70 of the external charger 50 typically comprises an on/off switch 72 that activates the charger to produce power 64, an LED 74 to indicate the status of the on/off switch, and a speaker 76 for emitting a “beep” at various times, such as when the external charger 50 is not properly aligned with the IPG 10 or when charging has completed.
To provide efficient power transfer, i.e., good coupling, from coil 62 to coil 48, the coils 62 and 48 are preferably wrapped in planes that are substantially parallel during a changing session. Good coupling is also promoted when the coils 62 and 48 are as close as possible, and when the axes around which they are wound are aligned, i.e., when the coils 62 and 48 and centered. Good coupling increases the power 64 transferred from the external charger 50 to the IPG 10, which as well as being efficient, minimizes heating in the IPG 10 and the external charger 12. Proper coupling may also be required for data transfer between the IPG 10 and the external charger 12.
Because charging the battery 25 in the IPG 10 may some time, it is desired to hold the external charger 12 in close proximity to and in alignment with the IPG 10 during a charging session. Typically, this occurs using an external charger holding device 100, such as a belt 102, as shown in FIG. 3. The belt 102 fastens around the patient's waist, and can be secured by a fastening device 108, such as a buckle, clasp, snaps, Velcro, etc. The belt 102 can be adjustable to fit patients with different waist sizes. The belt 102 includes a pouch 104, which generally hangs from the belt 102 in a position where the IPG 10 is implanted in the patient's buttocks. A slot 106 or other opening in the belt 102 allows the external charger 50 to be inserted into the pouch 104, such that the external changer 50 is, like the pouch 104, generally aligned with the IPG 10. Once placed in the pouch 104, the patient can press the on/off switch 72 on the external charger 50 to begin a charging session, or the user can turn the charger on before inserting it in the pouch 104. Affixing the external charger 50 to the patient using belt 102 allows the patient to move or walk while using the external charger 50, and thus can charge his implant “on the go.” See also U.S. Patent Application Publication 2012/0012630, describing another belt for an external charger.
While an external charger holding device 100 such as a belt 102 performs suitably to generally hold the external charger 50 in alignment with the IPG 10 in an SCS application, the inventors have noticed certain shortcomings with this approach. First, belt-style holding devices may work well for implantable medical device implanted around the waist region, but are not generally suited for holding and positioning the external charger 50 at other locations in the body where devices can be implanted. The fastening means 108 can break or wear out. Belt-style holding devices also require two pieces—the external charger 50 and the belt 102—which the patient must keep track of. Additionally, belt-type holding devices may shift as the patient moves, which can require the patient to keep adjusting the position of the belt to achieve good alignment with the IPG 10.
Additionally, belt-style holding devices do nothing to address heating in the external charger 50. As discussed elsewhere, see, e.g., U.S. Patent Application Publications 2008/0027500; 2011/0234155; 2011/0178576; and 2011/0071597, the magnetic charging field generated by coil 62 tends to generate heat in the external charger 50. Such heating can occur when the magnetic field interacts with other conductive structures in the external charger 50, such as the PCB 54, the battery 52, and other electronic components 56. The magnetic field induces Eddy currents in such conductive structures, which will heat because of their resistance. Heating is an important consideration in an external charger 50, because it runs the risk of irritating or hurting the patient, particularly given that the external charger 50 is typically in contact with the patient. Unwanted coupling of power to conductive components in the external charger 50 further reduces the power 64 available for charging the IPG 10. While the above-cited publications discuss ways to address such concerns, belt-style holding devices by themselves do nothing to address such concerns, as they do involve any redesign of the external chargers themselves. In fact, the present inventors realize that by encompassing the external charger 50 in a pouch 104, such holding devices tend to exacerbate heating concerns, because the pouch 104 insulates the external charger 50 to some degree and thus doesn't permit heat to radiate away from the external charger.
Another prior art system 150 is shown in FIG. 4, and is disclosed in U.S. Patent Application Publication 2009/0118796, which is incorporated herein by reference, and with which the reader is assumed familiar. System 150 comprises an external controller 152 able to bi-directionally wirelessly communicate with the telemetry coil 42 (FIG. 1A) in the IPG 10. This is useful for example to allow a patient to change the therapeutic setting of his IPG 10 using a graphical user interface comprising a screen 154 and various buttons 156, or to monitor various data of interest from the IPG 10. In addition to this communicative function, the external controller 152 is also coupleable to an external charging coil assembly 160 containing a charging coil 162. The external controller 152 contains electronics and programming for energizing the charging coil 162 with an AC current, thus producing a magnetic charging field for charging the IPG 10. That is, by attaching the external charging coil assembly 160 to the external controller 152, the system 150 becomes in effect an external charger, controlled using the external controller 152's user interface and circuitry. When charging of the IPG 10 is unnecessary, the external charging coil assembly 160 can be detached from the port 164 on the external controller 152, which can now resume its normal function of communicating data with the IPG 10.
The combined external controller 152 and external charging coil assembly 160 is beneficial for the reasons stated in the '796 Publication. Furthermore, and although not discussed in the '796 Publication, the present inventors recognize this prior art system is beneficial from a heating perspective. Because the conductive structures in the external controller 152 (a PCB, a battery, etc.) are distant from the charging coil 162, the magnetic field produced by the charging coil 162 will not significantly induce Eddy currents in such structures. The present inventors realize that this reduces heating in the system 150, and reduces power loss to such components.
Still, the system 150 still has to be affixed to the patient during a charging session. The external charging coil assembly 160 is attached to the external controller 152 by a cable 166 comprising wires. Thus, even if the patient is holding the external controller 152 portion of the system in his hand, or has put the external controller 152 is his pants pocket for example, the external charging coil 162 would still have to be affixed to the patient to hold it into alignment with the IPG 10. Thus, and although not discussed in the '796 Publication, at least the external charging coil assembly 162 (and possibly also the external controller 152) would still need to be inserted into a belt type-holding device such as shown in FIG. 3, particularly if the patient wants to move or walk while charging. This is inconvenient for the reasons stated above.
An improved design for an external charger for an implantable medical device, and an improved means for affixing the external charger to a patient during a charging session, is therefore desired. It is further desired that such improved design be able to charge implantable medical devices wherever they are implanted in a patient.